In a prior-art technology, an EGR (Exhaust Gas Recirculation) system is provided in order to suppress NOx (nitrogen oxides) low in an exhaust gas of an internal combustion engine such as a diesel engine. At this time, a detectable physical amount is fed back and a mass flow rate of the recirculating EGR gas is adjusted by controlling an EGR valve provided in an EGR passage so that the physical amount matches a target value of the physical amount determined in advance.
Thus, there is a device which converts a target exhaust recirculating amount set as a mass flow rate to a volume flow rate and determines a target area of the exhaust recirculating path on the basis of the target exhaust recirculating amount as the volume flow rate (see Patent Document 1, for example). This device can set the mass flow rate of the exhaust gas in accordance with an operation condition and can achieve highly accurate control of the target area of the exhaust recirculating path.
Since the volume flow rate of the EGR gas is in proportion to ½ power of a before and after differential pressure in a certain passage area of the exhaust recirculating path, the above-described device acquires the target area that can obtain the target exhaust recirculating amount on the basis of the target exhaust recirculating amount as the volume flow rate and a differential pressure between an intake pressure and an exhaust pressure.
In the current technology, it is difficult to directly detect the mass flow rate of the EGR gas and thus, the mass flow rate of the EGR gas is calculated with a control logic of an internal calculation model which acquires an engine intake/exhaust system state amount by calculation as in the above-described device.
Usually, since the mass flow rate of the EGR gas passing through the EGR valve can be handled as an isentropic flow of a nozzle, it can be expressed by Formula (8) below.
                    [                  Formula          ⁢                                          ⁢          8                ]                                                                      m          egr                =                              μ            ·            A            ·                                          P                in                                                              R                  ·                                      T                    in                                                                                ⁢                                                                      2                  ⁢                                                                          ⁢                  k                                                  k                  -                  1                                            ·                              [                                                                            (                                                                        P                          out                                                                          P                          in                                                                    )                                                              2                      k                                                        -                                                            (                                                                        P                          out                                                                          P                          in                                                                    )                                                                                      k                        +                        1                                            k                                                                      ]                                                                        (        8        )            
Here, it is assumed that megr is a mass flow rate ([kg/s]) of the EGR gas passing through the EGR valve, μ is a flow rate coefficient ([-]), A is an effective opening area ([m2]) of the EGR valve, Pin is a pressure at the EGR valve inlet ([Pa]), Pout is a pressure at the EGR valve outlet ([Pa]), Tin is a temperature at the EGR valve inlet ([K]), R is a gas constant ([J/kgK]), and k is a specific heat ratio ([-]).
By experimentally acquiring μ·A obtained by multiplying the flow rate coefficient μ of the EGR gas by the effective opening area A of the EGR valve (hereinafter referred to as a coefficient of an isentropic flow of the nozzle), it can be expressed as a function of an EGR valve opening degree xegr and an inter-valve differential pressure Pin−Pout by Formula (9) below.[Formula 9]μ·A=ƒ(xegr,Pin−Pout  (9)
This function is substantially given as an interpolation value from a map acquired from an experiment result in advance.
Depending on an engine specification, a differential pressure between an inlet and an outlet of the EGR valve is small and becomes negative depending on an operation state. In such an engine, a reed valve (check valve) is provided in the EGR passage in order to prevent a backflow of the EGR gas and also, the EGR gas is pumped out by using periodic differential pressure fluctuation generated by pulsation of a cylinder intake/exhaust stroke.
In the state in which the differential pressure between the inlet and the outlet of the EGR valve is small or becomes negative as above, the reed valve effectively works and a flow of the EGR gas is generated by a pumping operation of pumping out the EGR gas and thus, the mass flow rate cannot be expressed by the isentropic flow of the nozzle and cannot be calculated by the above-described formula (8).
Moreover, the above-described method in Patent Document 1 is also a method for control on the basis of the target exhaust recirculating amount as the volume flow rate and a differential pressure between the intake pressure and the exhaust pressure, but since its condition is that the mass flow rate of the EGR gas can be expressed by the isentropic flow of the nozzle, it cannot be applied to the internal combustion engine provided also with the reed valve.